{ INFORMATICS.MCCME.RU
  Problem statement: http://informatics.mccme.ru/moodle/mod/statements/view3.php?chapterid=320
  Verdict: Accepted
}
uses SysUtils, Math;
 
var
	i,n:longint;
	ans:extended;
 
begin
reset(input,'input.txt');
rewrite(output,'output.txt');
read(n);
ans:=1;
for i:=2 to n do
	ans:=ans+1/(int64(i)*int64(i));
write(ans:0:9);
end.